UPDATE: Apple released iOS 4.3.4 that is only tethered jailbreakable. Apple has stopped signing iOS 4.3.3, so you can NOT restore to 4.3.3 any more (both original and custom), unless you have saved your SHSH keys for 4.3.3.

If you are on 4.3.3 already - jailbreak with redsn0w or jailbreakme.

Proceed only if you have iOS 4.3.3 SHSH keys saved.

Step One
Make a folder called "Pwnage" on the desktop. Download a couple of things:

When downloading the IPSW file, it is best to download it with Firefox since Safari often auto extracts it!

Step Two
Double click to mount PwnageTool then drag the PwnageTool icon into the Pwnage folder.

Then from the Pwnage folder double click to launch the PwnageTool application.

Click Ok if presented with a warning.

Step Three
Click to select Expert Mode from the top menu bar

Step Four
Click to select your device. A check-mark will appear over the image of the device. Click the blue arrow button to continue.

Step Five
You will be brought to the "Browse for IPSW" page. Click the Browse for IPSW... button.

From the popup window select your iOS 4.3.3 firmware from the Pwnage folder then click the Open button.

Step Six

You will then be brought to a menu with several choices. Click to select General then click the blue arrow button.

The General settings allows you to decide the partition size. Check Activate the phone if you are not with an official carrier then click the blue arrow button.

The Cydia settings menu allows you to create custom packages so you do not have to manually install the necessary them later. If you do not want to install anything here go to next step.

Click to select the Download packages tab. Then click the Refresh button to display all the available packages. Double clicking the package you want will download it and make it available in the Select Packages tab.